SPINEL. 88 (2.) These truncations sometimes increase so much that they form a three-planed acumination, the apex of which is frequently truncated. (3.) When the prism of the preceding figure becomes shorter, and at length disappears, (if the acumination is obtuse,) there is formed a flat, double, three-sided pyra mid, in which the lateral planes of the one are set on the lateral planes of the other, and the remains of the six- sided prism form truncations on the common base. If the acumination is acute, there is formed a nearly cu bical figure, having two diagonally opposite angles trun cated. (4.) When the apices of the preceding crystals are deep ly truncated, a table is formed. Internally, it is shining. The fracture is conchoidal, and also concealed folia ted, parallel with the terminal planes of the prism. It is generally only translucent, and exhibits a pearly light on its surface. It is somewhat heavier than true spinel; but in other characters agrees with it. It occurs principally in the peninsula of India. M. Verina is of opinion, that many of the rubies of commerce are varieties of this mineral. 3. Spinel was first established as a species by Rome de Lisle and Werner, and separated from sapphire, with which it had been confounded. Vol. I, C 4. Sapphire,